The present invention relates to a method for the control of a particle mass which is evaporated per unit time by a particle beam striking a target. The particle beam is specifically a beam of charged particles, and in particular an electron beam in which the size of the impingement area of the beam on the target material is varied.
In the evaporation of targets by means of electron beams during vacuum coating processes, it is important to be able to influence the particle mass per unit time which is evaporated from the target material.
It is known in this connection, from IEEE Transactions on Magnetics, Vol. MAG-11, No. 2, March 1975 "Electron Beam Evaporation Synthesis of A15 Superconducting Compounds: Accomplishments and Prospects" by R. H. Hammond, to drive the mass current of the generated electron beam by changing the heating current of an emission electrode in the beam generator. With changing mass current of the electron beam, corresponding changes in the masses of particles evaporate from the target area on which the electron beam impinges, also take place.
This approach is disadvantageous since there is a long reaction time between application of a changed heating current and the effect with respect to mass current of the electron beam. Changing heating current as a final control element thus behaves like a lowpass element with high time constant. It is known that such elements, in particular when used on control circuits, are extremely disadvantageous, because fast guidance signal changes cannot take place rapidly nor are disturbances of higher frequency controlled unless expensive compensation measures are taken. Control circuits with such final control elements consequently have extremely high inertia. In the above cited article limit frequencies of up to 1 Hz are given.
In order to eliminate this disadvantage, the above cited article further suggests controlling the dwelling time of the electron beam at a site on the target material. In this way, the degree of heating is controlled locally at the target. To this end, a relatively complex system is required to shift the electron beam back and forth with precise control at the target. Moreover, the evaporated particle mass depends not only on the dwelling time of the electron beam at one place on the target, but also on the prior thermal history which this place or its environment have experienced. Through such dwelling time control, an unsteady evaporation is obtained, since the vaporization point is moved back and forth. It is, however, a goal of an effective rate control scheme to carry out evaporation as steadily as possible.
The dependency between dwelling time and evaporated particle mass is consequently relatively complex.
With this approach used as the regulating method in a control circuit according to the above cited article, limit frequencies of up to 10 Hz can be achieved.
Furthermore it is known from Proceedings of the Eighth International Vacuum Congress, Sept. 22-26, 1980, Cannes, France, Volume II "Vacuum Technology and Vacuum Metallurgy", pp. 542, "Closed loop multi source evaporation rate control with a quadruple mass spectrometer in an ultra high vacuum system" to change the evaporated particle mass at a target which is evaporated by means of an electron beam, by changing the voltage applied to a so-called Wehnelt electrode in the system.
A Wehnelt electrode is essentially an electrode with an aperture through which the electron beam passes. If its electrical potential is changed, the passing electrons are subjected to a more or less radially acting force and the bundeling of the electron beam is changed. The electrons, however, are simultaneously accelerated or decelerated before passing through the electrode ring, depending on the applied Wehnelt potential. When decelerated, a corresponding percentage of the emitted electrons is reflected back to the emission electrode.
Therefore by changing the potential of the Wehnelt electrode the beam bundeling as well as the mass current of the beam is changed.
If the effect of a positive potential applied to the Wehnelt electrode is considered, for example, it is clear that the beam would be widened radially, but that simultaneously the beam mass current is increased through axial acceleration forces in the vicinity of the Wehnelt electrode.
The incident electron density per unit area on the target material is decreased, however, by the widening of the beam, leading to a decrease in the particle mass evaporated from the target, per unit time.
On the other hand, when the Wehnelt voltage of the electron beam mass current is increased, this obviously increases the electron density per unit area on the target. Increased density also increases the particle mass removed per unit time from the target material.
By changing the Wehnelt voltage, two contrary effects with respect to particle mass removal are thus necessarily experienced simultaneously. Although the transmission characteristic of changes in the beam bundeling with respect to Wehnelt voltage and beam mass current with respect to voltage are different, so that the effective end result always leads to a change in the evaporated particle mass, the total transmission between evaporated particle mass and applied Wehnelt voltage is not linear and the steepness of the regulating characteristic is only satisfactory in a relatively small regulating range. The utilizable approximately linear regulating range is relatively small. Due to its nonlinearity, this approach thus can only be used within narrow limits.